The Repeatability and Effect of Age on Yearly Breeding Efficiency of Sires used in Artificial Breeding

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Abstract
With the general acceptance and growth of artificial breeding, the problem of selecting and obtaining suitable sires has become increasingly difficult. Once a sire is added to the group being used by the breeding association, he represents a considerable investment and it is desired to receive as much service from him as possible. Florida workers reported recently (1) that the average life of a bull in arti- ficial breeding is 3 yr. and that 56 per cent of all sires are removed because of low conception. It is desirable, therefore, to determine the effect of advancing age and the repeatability of conception rates for single yearly average records. This will indicate the probability of a sire with a desirable conception rate continuing in service and repeating his previous record year after year. In the case of sires with undesirable conception rates, such information may justify early removal, thus eliminating costly maintenance of undesirable bulls. It was with this idea in mind that this study was initiated. No previous reports were found in the literature for repeatability of concep- tion rates. Several workers, however, have reported the effect of age as a factor influencing breeding efficiency. Bowling reported (2) a gradual decrease in breeding efficiency of bulls with advancing age, becoming significant at 6 yr. Tanabe (8) and Erb (3) both reported highest conception with 1- and 2-year-old bulls and a gradual decline with age thereafter. Hilder (5) agreed closely with other workers in the general decline of conception with age, but reported that 7-yr.-old bulls were an exception in the group of bulls studied. EXPERIMENTAL PROCEDURE